Photoreduction of (99)~Tc Pertechnetate by Nanometer-Sized Metal Oxides: New Strategies for Formation and Sequestration of Low-Valent Technetium

摘要

Technetium-99 (99~Tc) (β~-_max: 293.7 keV; t1/2: 2.1 × 10 years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral-basic environment, the pertechnetate anion ( TcO4~-) is stable. TcO4~- is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils, or sediments. This study moves forward a new strategy for the reduction of 99~TcO4~- and the chemical incorporation of the reduced 99~Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), α_2-[P_2W_17O_61] ~10-, that can be photoactivated in the presence of 2-propanol to transfer electrons to 99~TcO4~- and incorporate the reduced 99~Tc covalently into the a_2-framework to form the 99~Tc~vO species, 99~Tcv~O(α2-P_2W_17O_61)~7-. This occurs via the formation of an intermediate species that slowly converts to 99 ~Tc~vO(α_2-P_2W_17O_61)~7- . Extended X-ray absorption fine structure and X-ray absorption near-edge spectroscopy analysis suggests that the intermediate consists of a "Tc(W) α_2- species where the 99~Tc is likely bound to two of the fourW-O oxygen atoms in the α_2-[P_2W_17O_61]~10~- defect. This intermediate then oxidizes and converts to the 99~Tc~vO(α_2-P_2W_17O_61)~7- product. The reduction and incorporation of 99~TcO4~- was accomplished in a "one pot" reaction using both sunlight and UV irradiation and monitored as a function of time using multinudear nuclear magnetic resonance and radio thin-layer chromatography. The process was further probed by the "step-wise" generation of reduced α_2-P_2W_17O_61~12- through bulk electrolysis followed by the addition of 99~TcO_4~-. The reduction and incorporation of ReO_4~-, as a nonradioactive surrogate for Tc, does not proceed through the intermediate species, and Re V~O is incorporated quickly into the α_2-[P_2W_17O_61]~10- defect. These observations are consistent with the periodic trends of 99~Tc and Re. Specifically, Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and the mechanisms of the reduction and incorporation of 99~Tc into a material.